The present invention relates to an image forming apparatus such as a copying machine or a printer, and in particular, to an image forming apparatus wherein a skew of a transfer sheet is corrected for image forming.
In the image forming apparatus such as a copying machine or a printer, a skew of transfer sheet has been corrected by registration rollers before image forming. With respect to correction of this skew, there has been an attempt wherein a skew of transfer sheet is detected automatically, and a tilt of registration rollers is changed based on the detected skew, thereby, a skew of transfer sheet is corrected (for example, Patent Document 1).
However, actual transfer sheet has no orthogonality, and corner angles of a sheet differ, depending on how sheets are cut. As shown in FIG. 7 (a), for example, an angle of a certain corner of transfer sheet P is 90° and an angle of another corner is 88.5°. There is also an occasion where a corner has a different angle on each transfer sheet. The following is considered as a reason why angles of corners are different on a transfer sheet, or why a corner angle is different for each transfer sheet. For example, there is an occasion wherein, when transfer sheets are cut by different cutting devices in a cutting process, a corner angle of transfer sheet differs depending on which cutting device cut the sheet. Or, there is an occasion where a corner angle differs depending on the size of transfer sheet. There is further an occasion where a corner angle differs depending on the manufacturer of transfer sheet even when transfer sheets are in the same size, when the transfer sheets are made by different manufacturers.
If corner angles of transfer sheet differ as stated above, there is sometimes an occasion where an image formed on the front surface and an image formed on the rear surface do not agree in terms of the position each other, even when a skew of transfer sheet is corrected by registration rollers.
When forming an image on the front surface of transfer sheet, for example, one end (a leading edge) of the transfer sheet P is made to hit a nip portion of registration rollers 43 so that a skew of the transfer sheet P is corrected as shown in FIG. 7(b), and after that, an image is formed on the front surface of image forming section. After the image is formed on the front surface, in the case where an image is formed on a rear surface of the transfer sheet, the transfer sheet is reversed by reversing path, reversing and conveying roller and reversing and conveying path, and is sent again to image forming section. Since the transfer sheet is reversed as stated above, the other end (trailing edge) of the transfer sheet P is made to hit a nip portion of registration rollers 43 as shown in FIG. 7 (c), and a skew of the transfer sheet P is corrected. After that, an image is formed on the rear surface by the image forming section.
However, since a skew angle of the leading edge is different from that of the trailing edge on transfer sheet P as shown in FIG. 7 (a), an image formed on the front surface is deviated from an image formed on the rear surface by a difference equivalent to the difference between the angles as shown in FIG. 7 (c), and it has been difficult to align an image on the front surface with an image on the rear surface accurately.
It is further difficult to detect an angle deviation automatically because an amount of the angle deviation of transfer sheet is extremely small, and it is difficult for the image forming apparatus relating to the Patent Document 1 to correct an angle deviation of transfer sheet.
(Patent Document 1) Unexamined Japanese Patent Application Publication No. 6-234441